Despite the U.S. Public Health Service's goal of eradicating measles by 1982, measles remains a prominent public health concern with over 27,600 cases reported in 1990. Especially troubling is the fact that 20% - 40% of these cases occurred among persons who had been appropriately immunized. While there is some information available about vaccine coverage in the population, there are surprisingly few data available about the seroprevalence of immunity to measles. This proposed study will serve to fill many of the gaps in our knowledge about the community prevalence of immunity to measles. This study will collect data from two North American populations which have enjoyed nearly 100 percent vaccine coverage for the past decade. Drawing on samples from the two populations, this study will provide estimates of the prevalence of measles immunity in the Grenfell region of Newfoundland, Canada and Olmsted County, Minnesota for persons of all ages. This study will take advantage of a "natural" experiment by comparing the community impact of immunizing children against measles at 15 months of age (Olmsted County) as compared to 12 months (Newfoundland). These estimates will be refined for children aged 7 to 11 years in these two communities who would have received only the reformulated (post-1980) measles vaccine. Estimates of the seroprevalence of vaccine-induced immunity to measles among these children will be calculated based upon probability samples derived from school records in these two communities. The relationship between age at immunization and subsequent immunity will be assessed at the community level (comparing policy) and at the individual level (comparing actual age at immunization). Antibody measurements will be done using ELISA methodology. Finally, this study will prospectively measure the immunologic effect of re-vaccination among persons with vaccine failure (either primary or secondary). Using a nested case-control design then, this study will serve to generate hypotheses to explain why persons fail to mount appropriate immune responses following re-vaccination. In summary, this study will provide fundamental information about the seroprevalence of immunity to measles at the community level as well as important insights into the relationships between timing of immunization, subsequent immunity, and immunization failure. These data are important in formulating national policy to eliminate measles.